Field of the Invention and Related Art Statement
The present invention relates to a solid state image pick-up apparatus comprising at least one solid state image sensor having a number of light receiving elements arranged in matrix in main- and sub-scanning directions and an optical filter arranged in an incident light path to the solid state image sensor for separating ordinary and extraordinary light rays at least in the main-scanning direction, and more particularly to a solid state image pick-up apparatus in which moire fringes generated near a sampling frequency i.e. a spatial sampling frequency of the solid state image sensor as well as moire fringes generated at frequencies higher than twice the sampling frequency can be effectively removed, so that cross modulation between the moire fringes and an image signal can be also suppressed in an efficient manner.
FIG. 1 shows the distribution of the light receiving apertures of a widely used solid state image sensor. In FIGS. 1, H represents a horizontal direction corresponding to the main-scanning direction, V a vertical direction corresponding to the sub-scanning direction, X a dimension of a pixel aperture in the horizontal direction and Y denotes a dimension of a pixel aperture in the vertical direction. Further, P.sub.x expresses a pixel distance between successive pixels arranged in the horizontal direction H and P.sub.y represents a pixel distance between successive pixels aligned in the vertical direction H.
An optical image impinging upon the solid state image sensor is spatially sampled by the above mentioned aperture pattern at a sampling frequency f.sub.s. Upon the image sampling, when the maximum spatial frequency f.sub.max contained in the image is higher than the Nyquist frequency f.sub.n which is equal to a half of the sampling frequency f.sub.s, repetitive spectrum patterns are superimposed on each other as illustrated in FIG. 2. Generally the spatial frequency of the image of an object to be picked-up is limited by an optical lens system provided in the image pick-up apparatus. However, although incident image information having frequencies lower than f.sub.n can be restored from the image signal obtained by the spatial sampling, image information having frequencies f higher than the Nyquist frequency f.sub.n can not be recovered and is converted into components having difference frequencies (f'=f.sub.s -f) from the sampling frequency f.sub.s. These components having the difference frequencies are folded back into a frequency region lower than the Nyquist frequency f.sub.n. These components generate pseudo signals termed folded back distortion. When an image is reproduced by an image signal containing a lot of the folded back distortions, there are produced moire fringes in a region of higher spatial frequencies and the image quality is deteriorated to a large extent.
Moreover, in the usual solid state image sensor, the width X of the pixel aperture measured in the horizontal direction H is smaller than the space (P.sub.x -X) between successive pixels arranged in the horizontal direction as shown in FIG. 1, so that the above mentioned folded back distortion is liable to be increased.
In order to avoid deterioration in quality of the reproduced image due to the folded back distortion, it is necessary to suppress abruptly spatial frequency components higher than f.sub.n without decreasing the response in the frequency range lower than f.sub.n. To this end, it has been well known to transmit the optical image of an object through an optical low pass filter having a trap frequency near f.sub.s before sampling the object image. In general, the optical low pass filter is formed by one or more birefringent quartz plates. A single quartz plate has a response characteristic which resembles a cosine curve shown by a thin solid line A in FIG. 3. In FIG. 3 the horizontal axis represents a frequency f.sub.x normalized by the sampling frequency f.sub.s. When a single quartz plate is used, although the low frequency components, i.e. the base band components are not suppressed greatly, the above mentioned folded back distortion can not be removed, because the decay near the sampling frequency f.sub.s is still small and thus the decrease in the image quality can not be compensated for sufficiently.
In order to avoid the above explained drawbacks, various kinds of optical filters having a plurality of birefringent quartz plates have been proposed in, for instance, Japanese Patent Laid-open publications Nos. Kokai Sho 60-164719, 61-270985 and 61-270986. In known optical filters disclosed in these references, three birefringent quartz plates are arranged to separate ordinary and extraordinary light rays from each other in directions of +45.degree., 0.degree. and -45.degree., respectively with respect to the horizontal scanning direction.
In the first reference, i.e. Japanese Patent Laid-open Publication No. 60-164719, there is described an optical filter for use in combination with a solid state image sensor having one chip image sensing device and a mosaic filter applied thereon. In this known optical filter, the separation width of first and third quartz plates is set to ##EQU1## and that of a second quartz plate is set to P.sub.x. Then the response curve may be expressed by a broken line B in FIG. 3 and has two trap points at f.sub.n and f.sub.s. Such an optical filter has a larger suppressing function for frequency ranges near f.sub.n and f.sub.s than the single quartz plate, but the suppression near f.sub.s is not large enough from a practical view point and the suppression in the base band is too large so that desired image components are also reduced and the image quality is deteriorated.
In the above mentioned Japanese Patent Laid-open Publication No. 61-270986, there is disclosed an optical filter for use in a solid state image pick-up apparatus in which the input image is first separated into red, green and blue color images and these color images are received by three separate solid state image sensors, the positions of the apertures of the image sensing elements for receiving the green image being shifted by a half of the distance between successive pixels with respect to those of the image sensing elements for receiving the red and blue images. In such a solid state image sensor, the sampling frequency for the brightness signal is apparently increased twice. This method is usually termed the pixel shift method. In the known optical filter for use in such solid state image pick-up apparatus, first and third quartz plates have separation directions of .+-.45.degree. with respect to the horizontal direction and have a separation width of ##EQU2## and a second quartz plate operates to separate ordinary and extraordinary rays from each other in the horizontal direction by a separation width of P.sub.x. In this known optical filter, the separation distance viewed in the horizontal direction is equal to P.sub.x as in the case of the above mentioned known optical filter described in Japanese Patent Laid-open Publication No. 60-164719. Therefore, sufficient suppression near the sampling frequency f.sub.s could not be obtained and further the base band is suppressed by a relatively large amount.
In the Japanese Patent Laid-open Publication No. 61-270985, there is also described a known optical filter for use in the solid state image pick-up apparatus to which the above explained pixel shift is also applied, said optical low pass filter including first and third quartz plates for separating ordinary and extraordinary light rays from each other in directions which make angles of .+-.45.degree., respectively with respect to the horizontal direction by the separation width of 2P.sub.x, and a second quartz plate for separating ordinary and extraordinary light rays from each other in the horizontal direction by a separation width equal to 1/2P.sub.x. In such a known optical filter, the trapping function at f.sub.n is improved more or less compared with the above mentioned known optical filters, but the suppression effect near f.sub.s is not so large and the base band is still suppressed too much.
In the known optical filters using three quartz plates so far explained, the filters are designed under the common conception that the amount of suppression in the frequency range higher than f.sub.n is increased by forming the trap points at f.sub.n and f.sub.s. However, such a known designing conception could not yield sufficiently large suppression near f.sub.s and deterioration of the image quality due to moire fringes could not be compensated for sufficiently. Further, the low frequency components are reduced to an undesired extent so that the resolution of the reproduced image is decreased. Particularly, for solid state image pick-up apparatus using the pixel shift method, although the sampling frequency for the brightness signal can be apparently increased twice, when the object includes a black and white pattern having frequency components near the sampling frequency, a so-called color moire is introduced and the reproduced image is colored in an undesired manner. For instance, when a white portion of the black and white pattern is made incident upon the green pixel and a black portion is made incident upon red and blue pixels, the black and white pattern is reproduced as a green pattern. The known optical filter could not remove such color moire fringes sufficiently.
The inventors of the present application have proposed, in Japanese Patent Application No. 63-160849, corresponding to Japanese Laid-open Publications 60-164719 and 2-13086 an optical filter in which components near the sampling frequency are sufficiently suppressed to avoid effectively the generation of moire fringes and decrease in the resolution is avoided by making the suppression in the base band as small as possible. Such an optical filter is particularly suitable for sufficiently suppressing the above mentioned color moire fringes in a solid state image pick-up apparatus adopting the pixel shift. This optical filter comprises three quartz plates for separating ordinary and extraordinary light rays in the incident light at least in the main-scanning direction such that the total response characteristic in the main-scanning direction has one trap point at or near the sampling frequency, but has no trap point in the frequency range below the sample frequency.
As explained above, in the known optical filter comprising a plurality of quartz plates, the design conception is based on the fact that the decay in the frequency range above f.sub.s is increased by producing a large number of trap points, but in the above mentioned optical filter developed by the inventors of the present application only one trap point is produced even though a plurality of quartz plates are used. That is to say, in the latter optical filter, three quartz plates are arranged such that the direction for separating the ordinary and extraordinary light rays from each other is set to .+-.45.degree. and 0.degree. so that the separation widths viewed in the main-scanning direction of all the quartz plates are made equal to 1/2P.sub.x. That is to say, first and third quartz plates are so formed that the separating direction is set to .+-.45.degree. and the separation widths are ##EQU3## and a second quartz plate is arranged such that the separation direction is set at 0.degree. and the separation width is set to 1/2P.sub.x. By arranging the three quartz plates in the manner explained above, it is possible to obtain only one trap point at the sampling frequency and the components near the sampling frequency can be suppressed greatly and the undesired decay in the base band can be decreased.
It has been confirmed that the optical filter described in the above mentioned Japanese Patent Application No. 63-160849 has no function to suppress moire fringes producing at higher harmonics of the sampling frequency f.sub.s, i.e. 2f.sub.s, 3f.sub.s, 4f.sub.s, 5f.sub.s as shown by a dotted line C in FIG. 3. In order to reduce such moire fringes it is necessary to arrange one additional quartz plate, so that the whole construction of the optical filter might be complicated and large and the cost of the optical filter is increased. A chain line D in FIG. 3 shows the response characteristic of such an optical filter having the additional quartz plate. The above mentioned moire fringes at higher harmonics of the sampling frequency are often generated in dot-printed matters.